The present invention relates to an optical storage medium in or from which data is recorded, erased or reproduced with irradiation of a light beam (for example, a laser beam), and also an optical recording method and apparatus for such an optical storage medium.
Phase-change optical storage media are data-rewritable storage media utilizing a reversible change phenomenon between a crystalline phase and an amorphous phase, such as, modern CD-RW, DVD-RW and DVD-RAM. Especially, DVD-RW and DVD-RAM are used for recording and rewriting a large capacity of data, such as video data. What are required for phase-change optical storage media now increasingly popular for video recorders, personal computers, etc., are high-density recordability for long-time recording and storage of a large capacity of data, in addition to recording and overwrite characteristics.
One particular technique that allows a larger storage capacity for phase-change optical storage media is to make physically smaller a signal to be recorded to raise recording density per unit of area. Known methods for such a smaller signal are, for example, shorting a wavelength of a laser beam for recording and increasing an aperture ratio (NA) for an objective lens.
An example using such methods is a Blu-ray disc with 23 GB/surface area in recording density compared to 4.7 GB/surface area for DVDs. A recording/reproducing apparatus for Blu-ray discs employs a 405 nm-laser beam with 0.85 in NA for an objective lens different from a 660 nm-laser beam with 0.60 in NA for DVDs.
Another technique that allows a larger storage capacity for phase-change optical storage media is a multi-layered structure (multi-layered storage medium) in which several recording films are stacked. Already in the market is DVD-ROM with a dual-layer structure of two recording films. The same idea has been applied to write-once and rewritable types (DVD-R, DVD-RW, etc.) to achieve about double and quadruple recording density for dual and quadruple layers, respectively.
Also already in the market are write-once dual-layer storage media having two organic-resin recording films, such as, DVD+R and dual-layer (DL) discs.
Such high density multi-layered storage media accept a long-wavelength laser beam and a conventional low-NA objective lens, thus recordable and reproducible with a conventional pick-up, which achieves low prices for recording/reproducing apparatus.
Discussed below are a known phase-change optical storage medium (single-layer optical storage medium) with a single rewritable recording film and a recording method for such a storage medium.
A known single-layer optical storage medium has a structure in which at least a dielectric film, a recording film, another dielectric film and a reflective film are stacked in order on a substrate having a bottom surface to be irradiated with a laser beam carrying a recording or reproducing power, or an erasing power.
In recording, recording pulses are applied (emitted) onto a recording film with a laser beam having a specific power, to melt and rapidly cool down the recording film, thus forming amorphous recorded marks thereon.
Reflectivity of the recorded marks lower than that of the crystalline-phase recording film allows optical reading of the marks as recorded data.
In erasing the recorded marks, a laser beam having a constant power (erasing power) lower than the recording power is emitted onto the recording film. The laser beam raises the temperature thereof to the crystallization temperature or higher but lower than the melting point to change the recording film from the amorphous phase to the crystalline phase to erase the recorded marks, thus overwriting being enabled.
A basic structure of a multi-layered phase-change optical storage medium is such that two or more of layer structures are provided on a substrate. Each layer structure has at least a dielectric film, a recording film, another dielectric film and a reflective film, like the single-layer optical storage medium discussed above.
In the following discussion, a layer structure having a recording film in addition to several kinds of films is referred to as a data layer.
Japanese Unexamined Patent Publication No. 2003-45085 discloses a phase-change optical storage medium having two or more of phase-change data layers each including a recording film. Excellent erasability is achieved with adjustments to the thickness and thermal conductivity of a protective film and a heat dissipation film that constitute each data layer, in addition to the recording film.
Confirmed by the inventor of the present invention in this phase-change optical storage medium are: comparatively acceptable C/N for 3T (unit clock cycle)-marks, whereas unacceptable overwrite characteristics (under adjustments to the thickness and thermal conductivity of the protective and heat dissipation films) to recording with a modulated signal which is actually used in data recording, such as, a random signal under 8-16 modulation.
The inventor of the present invention examined advantages discussed in Japanese Unexamined Patent Publications No. 2003-6923 (adjustments to the thermal conductivity of the protective film at 50 W/m·deg or higher) and No. 2003-242687 (selection of cubic boron nitride for the heat dissipation film formed on the reflective film that constitute a data layer closer to a laser-incident surface). Nevertheless, both were unacceptable in random-signal recording discussed above.
Japanese Unexamined Patent Publications No. 2003-22572 evaluates random-signal recording with Al, Si and N as elements of a material of the protective film. Nevertheless, the inventor of the present invention confirmed unacceptable recording and overwrite characteristics.
The inventor of the present invention employed a particular recording pulse sequence in evaluation of the multi-layered phase-change optical storage media disclosed in the above Japanese Unexamined Patent Publications. This is the same as a known recording pulse sequence used in recording to single-layer phase-change optical storage media.
This known recording pulse sequence is referred to as a recording pulse sequence 1. Illustrated in FIG. 1 is a recording pulse sequence 1 for use in forming 3T and 5T recorded marks, which is disclosed, for example, in Japanese Patent No. 3266971. The sequence 1 is modulated as having a high power (Pw) pulse and a low power (Pb) pulse.
The inventor of the present invention also employed the recording pulse sequences under the 2T-multi pulse strategy defined in DVD-RW Version 1.2 and the non-multi pulse strategy defined in DVD-RAM 5×-SPEED Optional Book, and confirmed unacceptable recording and overwrite characteristics particularly on data layers, except for the one most remote from the laser-incident surface.
Recording/erasure to/from phase-change optical storage media controls a laser beam to heat/cool recording films. Under the laser control, multi-layered phase-change optical storage media require data layers, except for the one most remote from the laser-incident surface, to be highly transparent to a laser wavelength for recording/erasure.
The former data layers and the latter one are referred to as highly transparent data layers and the furthest data layer, respectively, in the following discussion.
Such highly transparent data layers have a thinner recoding film and also a thinner reflective film than the furthest data layer.
The inventor of the present invention confirmed unacceptable recording and overwrite characteristics on those highly transparent data layers under a known recording technique with the same recording pulse sequence as for the furthest data layer.
As discussed above, the multi-layered phase-change optical storage media, having the highly transparent data layers with thinner recoding films and thinner reflective films than the furthest data layer, suffer unacceptable recording and overwrite characteristic under the known recording technique.